You Can Get Very Lucky While Leveling And End Up At 1Hp, Then Use A Potion.
(This Is What I Did)
Or
You Can Duel Someone, Losing A Duel (While Staying In The Duel Circle) Leaves You At 1Hp.
- Smash The Potion Button Before Life Regen Kicks In And The Achievement Is Yours.
Good Luck!